1. Field of the Invention
This invention relates to a method and apparatus for the automatic detection of cutting tool wear, and more particularly, to the automatic detection of worn cutting tools using statistical analysis techniques of the vibrational frequencies emanating from a cutting tool-workpiece interface and/or other working characteristics of a machine tool in the machining process to determine when the characteristics of an excessively worn cutting tool have been reached.
2. Technological Background
In performing machining operations on a workpiece in a machine tool such as a lathe, milling machine, or planer, the rate of wear of the cutting tool edge depends largely upon the workpiece material and machining conditions such as feed, speed, and depth of cut. Thus, the rate of wear can vary within wide limits. It is generally considered to be economical in production machining to operate the machine tool to achieve high metal removal rates, which means that the cutting tool wears out rapidly. The reason for this is that operator time and machine use time are expensive, whereas the cutting tool insert is relatively low priced.
Metal cutting tools usually reach the end of their useful lives via a wearout mechanism, rather than via an abrupt tool fracture. The accumulated cutting time before a tool should be discarded or reworked, even with a fixed cutting task, varies by a large factor, sometimes as large as 2 to 1, because of variability in quality among tools in a given batch, and because of large variations from batch to batch. Therefore, if no operator is present to detect and replace tools as they become worn out, it is necessary to schedule tool changes on the basis of the shortest life expected from any tool in the batch. The result is a tool budget far higher than would be necessary if each tool could be used to near the end of its own unique useful life. Thus, to prevent tool budget increases from reducing the savings in automation of the machining process that would otherwise result from reduced labor costs, a tool wearout indicator is needed.
However, as the tool wear rate increases, it becomes increasingly important to know exactly when to replace the worn cutting tool. If the cutting edge is replaced too soon, the operation is not economical, and if it is replaced too late and the workpiece is machined with an excessively worn cutting edge, this may result in incorrect dimensions and degrade the surface finish. Therefore, it is usually considered necessary to manually watch the cutting edge condition, even during an otherwise completely automatic machining operation, and manually replace the cutting insert when necessary. It is also possible to use an automatic insert changer, which can be set to replace each insert after a certain cutting time. However, all cutting tools do not fail after the same time interval, but over a range of time, and consequently the insert changer must be set to make the change at a shorter time interval to reduce the risk of running inserts which have failed. Most inserts will then be replaced before it is actually necessary, and optimum economy is not achieved.
By way of definition, the term "vibration" as used herein means any mechanical wave displacement, velocity or acceleration emitted by a tool when in use and encompasses the phenomenon known as "acoustic emission".
The term "machining conditions" as used herein refers to controlled factors in which, in a machining operation using a cutting tool, changes could give rise to substantial alterations in the tool vibration during machining, and specifically including cutting tool type, tool holder, cutting speed, feed rate, depth of cut, workpiece material, and amount and type of coolant. A change in one of these factors would result in a change in machining conditions.
One method utilized in the art to determine the extent of cutting tool wear has been a sonic worn cutting tool detection technique, such as that described in commonly assigned U.S. Pat. No. 3,548,648. The aforementioned patent utilizes a sonic energy transducer to convert the sonic vibrations to a continuous analog electrical signal, separating the analog signal into high and low frequency components in the range of 4-8 kHz and 0-4 kHz respectively, and comparing the analog signal high and low frequency components to each other to determine the extent of wear or condition of the cutting tool. When the ratio of the high and low frequency components reaches a certain predetermined level, an output signal is generated which is indicative of cutting tool wear.
In U.S. Pat. No. 3,548,648, the comparison of analog signals and the integrating process disclosed therein is time consuming and not as accurate in computing as would be possible in a digital mode. The ranges of vibrational frequencies utilized by the device disclosed in that patent are in the sonic range. Tool wear in the device was indicated by an increase in the high frequency energy level relative to the low frequency energy level.
In the aforementioned patent, the vibration frequency bands used include some of the resonant mechanical vibrational frequencies of the tool holders used to clamp the tool into place in the machine. These relatively low resonant frequencies could be easily excited by machinery noise not associated with the condition of the cutting edge of the tool. The tool holder resonant frequencies thus generated by machinery noise could lead to spurious results and premature or late indications of cutting tool wear. The levels indicative of cutting tool wear thus needed to be changed each time a change was made in the machining conditions. Machining conditions which affect the vibrational energy signature are the type and precise composition of the insert material; the shape of the insert and other geometry factors; methods of mounting the insert in the tool holder including material and geometry of the tool seat and use of a chip breaker; chatter; depth of cut, feed rate and spindle speed; roughness of the workpiece surface, including surface scale and previously machined holes; workpiece material; and cut discontinuities at the inside and outside corners.
Another method utilized in the field of tool wear detection utilizes a sensor for sensing the amount of force needed to continue machining a workpiece during the cutting operation. This method suffers from certain drawbacks, among these being a difficulty in installation, the need of extensive modification to the tool holders, and their great expense. Moreover, these devices are also sensitive to changes in the machining conditions, usually to the same degree or greater than their sensitivity to tool wear. Thus, the devices utilized by this method are not as accurate as may be desired.
As opposed to the above methods, the present invention uses a simple combination of a high frequency vibration signal sensor with another signal derived from the machining process that is indicative of the power used to machine a workpiece.
The present invention is distinguished from that of U.S. Pat. No. 3,548,648 in the use of a much higher (25 kHz to 100 kHz) high frequency vibration band, in relating tool wear to a decrease, rather than an increase in high frequency vibration energy; in lack of dependence upon mechanical resonances in a particular tool and tool holder arrangement; and in lack of sensitivity to changes in machining conditions.